Samsung and MXJO's end voltage is 2.5 volts.
Below 2.5 volts and the cells with have irreversible cell death, not allowing you to charge them.
3.5-3.6 resting voltage is the voltage you could consider "charge state". Meaning it is the best time to recharge the battery as the cell can no longer produce the 3.7 voltage rating of the battery. Regulated mods combat this by pulling more amps from the battery.
Truth be told, you might get longer performance from a battery by pulling it as soon as it goes below 3.7 volts. But by constantly pulling the battery at this voltage and using up a charge cycle ... you can also introduce increased resistance over time. Every single battery cell is slightly different and can react differently so there is no exact science.
Since batteries are relatively inexpensive, I choose to not pay much attention to this matter. I figure if I can get 9-12 months of life out of a battery I've received my moneys worth. My methods work for me, but someone else might feel different.
As long as you aren't going past a batteries end voltage rating (and allow some wiggle room) ... you should get enough use out of it before needing to replace it IMHO.
Thanks for the valuable input!